1. Field of the Invention
The present invention relates generally to atherectomy catheters and more particularly to a catheter having a cutting apparatus for cutting along two different axis.
2. Previous Art
The high incidence of disease and death from atherosclerosis in the general population has stimulated intense interest in devices and procedures for treatment. Atherosclerosis is a disease of the circulatory system caused by excess internal tissue growth or fatty or calcified deposits within blood vessels in the human. These deposits are generally characterized as stenoses. Stenoses tend to reduce the cross sectional area of the blood vessels which results in reduced blood circulation. Severe cases of blockage can cause or contribute to myocardial infarction, stroke and other conditions.
Procedures have been developed for treatment of these conditions, such as coronary bypass surgery. The expense and risk of bypass surgery has stimulated the use of alternative treatments such as balloon angioplasty, wherein a balloon catheter is positioned within a blood vessel adjacent to a blockage and inflated to compress or displace the blockage. The expense and risk of balloon angioplasty is substantially less than for bypass surgery but may be ineffective if the blockage is asymmetrical, highly calcified or composed of highly fibrous tissue. There is also some risk of restenoses since the stenotic material, which remains in place in the vessel, can grow back.
Atherectomy devices for clearing or opening blood vessels obstructed by stenotic material have been developed to counter these disadvantages and are well known. In an atherectomy procedure, the stenotic material causing the obstruction in the vessel is cut or otherwise separated from the interior of the vessel and removed.
A number of atherectomy devices have been disclosed. An example of such a device is U.S. Pat. No. 4,020,847 to Clark which discloses a blunt hollow tube on the end of a flexible catheter. A cutting edge on side of a lengthwise window opening in the tube is used to cut stenoses from a vessel by rotating the tube in a cutting direction defined by the cutting edge. Stenotic tissue intruding or invaginated into the opening is subjected to a cutting action tending to pull the tissue in the direction of rotation and separate it from the tissue outside the tube. A disadvantage of this structure is the tendency of the separated edge to be rough due to the distortion induced in the tissue as it reacts to the pulling action of the rotation of the tube and cutting edge. Also, the angular extent of the window opening limits the amount of tissue invaginated into the housing thereby limiting the depth of cut. A small opening requires multiple cuts to achieve a desired depth of cut. A large opening increases the distortion of the tissue and roughness of the cut. The shape of the cut tends to be non-uniform due to the tissue distortion caused by the cutting force directed normal to the cutting window edge.
Another example of an atherectomy device having a rotary cutter is shown in U.S. Pat. No. 4,986,807 to Farr. This discloses a blade having a rotating cutting edge capable of projecting beyond the diameter of the housing containing the blade. This allows the device to make a cutting area larger than that possible without such projection. The uni-rotational rotary cutting action of the blade tends to distort the tissue as it cuts leaving a scalloped cut of non-uniform thickness. In addition, there is some risk of perforation of the blood vessel wall since the cutting edge does project beyond the housing wall.
An example of an atherectomy device having combined rotary and axial cutting motion is disclosed in U.S. Pat. No. 5,156,610 to Reger. Several blades are mounted in a helical basket configuration and spaced angularly apart from one another about the associated ends of two concentric sheaths in such a way that longitudinal and rotary relative movement of the sheaths selectively bows the blades arcuately outward into a cutting position or draws the blades fiat into alignment with the sheaths. The degree of bowing determines the diameter of the cutting action.
Removal of cutaway pieces of the stenoses is accomplished by either pull-back of a balloon embolectomy catheter or by use of a latex membrane enshrouding the blades and used to trap the tissue shavings within the membrane. The diameter of the cut is not limited by a housing surrounding the cutting blades thus increasing the possible risk of vessel perforation if cutting is too aggressive. The mechanism is a complicated and delicate assembly.
Another example of an atherectomy catheter having a rotatable and axial translatable cutter is disclosed in U.S. Pat. No. 5,154,724 to Andrews. An expandable cutter head having spaced apart blades radially extending from a cutter sleeve at the distal end of the cutter head is axially extended from a guiding catheter inside a blood vessel. A torque tube and expander cable within the catheter control the radius of the cutter head blade expansion. The outermost radius of the blades at the cutter head determine the depth of tissue cut from the vessel wall.
A vacuum applied to the proximal end of the torque tube is used to draw off the material cut away by the cutter. The effectiveness of the vacuum for removing material is limited by the small diameter of the lumen required for small blood vessels and the long path along the catheter.
It is difficult to properly direct and stabilize such a cutter. The depth of the tissue cut from the vessel wall is dependent on the skill and aggressiveness of the operator. There is no safeguard provided to limit the cutting depth as the blades are extended, hence cutting must be limited to a passageway well removed from the vessel wall. Such an approach would not be effective for hard deposits, as it would be difficult to provide sufficient force to cause the blades to press in a radial direction.
The control problem is addressed by a second approach. In this approach the catheter has a cutter window disposed along the side of the catheter. A balloon is used to stabilize the catheter and to mechanically force the window against the deposits, causing some of the deposits to enter the housing itself. A rotating circular cutting blade is advanced longitudinally parallel to the catheter axis, thereby cutting deposits which extend through the window into the housing. The extension of material in this manner is termed invagination. An example of an atherectomy catheter illustrating this approach is in U.S. Pat. No. 4,979,951 to Simpson. This discloses a housing containing a rotating cup-like cutter which moves past a window opening in the housing. Tissue invaginated in the opening is cut by the axial motion of the cutter.
This approach has certain limitations. First, the depth of such a cut and the amount of material removed is limited by the angular width of the window. The width of the window is limited to that required to safely hold the cutter in the housing. The diameter is limited to the sheeth size selected for a given artery.
Secondly, The cut is not uniform along the axial direction. The cutting force is directed primarily to the bulk of the invaginated tissue. When the cutter first encounters the invaginated tissue, it cuts and engages the material. As the cutter is advanced, it pulls on the material, drawing more through the cutter window. This ususally causes a distortion of tissue as the cut begins thin and thickens as it proceeds to the terminal end of the cut. This can result in a steep slope at the back end of the cut as the cutter passes the end of the window. The resulting teardrop-shape of the axial cross-section leaves a nonuniform depth along the extent of the cut.
Thirdly, the limited angular width of the window opening which prevents the cutter from leaving the housing, also tends to cause a scalloped cut in the circumferential direction. Scalloped cuts tend to cause the housing to orient preferentially into the trough created by the scallop. This makes it more difficult to extend the tissue removal in a smooth angular manner by doing multiple cuts closely juxtaposed.
For each of the devices disclosed in these references, the cutting direction is primarily at right angles to the cutting edge. Consequently, the tissue being cut experiences a force tending to distort the tissue shape so that the cutting depth is nonuniform along the extent of the cut. Steep edges at the separation point tend to cause the cutter housing to preferentially locate in the previous cut such that repetitive cuts increase thickness non-uniformity.
Accordingly, there is a need for an atherectomy device which provides for a defined tissue cut with a generally uniform depth across the extent of the cut. Also the cut needs to have a relatively shallow incline with a well defined edge at the separation of the cut stenotic tissue from the remaining tissue to prevent the housing from preferentially locating in a previous cut.
In addition, there is a need for receiving the cut tissue and preventing it from being reintroduced into the blood vessel.